1. Technical Field
This invention relates to methods and devices for measuring the thermal characteristics of fluids. Specifically, this invention relates to the methods and devices for measuring the thermal oxidation tendencies of fuels used in liquid hydrocarbon-burning engines.
2. Background Art
When engines were developed for use in jet aircraft, problems began to develop for jet fuel due to poor fuel thermal stability. At higher temperatures, the jet fuels would oxidize and form deposits that would clog fuel nozzles and fuel filters. These deposits would also collect in the jet engine.
While various tests were devised and used in the 1950s and 60s to rate the thermal oxidation characteristics of jet fuels prior to being used in jet aircraft, Alf Hundere developed the apparatus and method which became the standard in the industry. In 1970, Alf Hundere filed what became U.S. Pat. No. 3,670,561, titled “Apparatus for Determining the Thermal Stability of Fluids”. This patent was adopted in 1973 as ASTM D3241 Standard, entitled “Standard Test Method for Thermal Oxidation Stability of Aviation Turbine Fuels”, also known as the “JFTOT® Procedure”. This early Hundere patent was designed to test the deposition characteristics of jet fuels by determining (1) deposits on the surface of a heater tube at an elevated temperature and (2) differential pressure across a filter due to collection of particulate matter. To this day, according to ASTM D3241, the two critical measurements are still (1) the deposits collected on a heater tube and (2) differential pressure across the filter due to the collection of particulate matter on the filter.
According to ASTM D3241, 450 mL of fuel flows across an aluminum heater tube at a specified rate, during a 2.5 hour test period at an elevated temperature. Currently six different models of JFTOT®1 instruments are approved for use in the ASTM D3241-09 Standard. The “09” refers to the current revision of the ASTM D3241 Standard. 1 JFTOT is the registered trademark of Petroleum Analyzer Company, LP.
While over the years various improvements have been made in the apparatus to run the tests, the basic test remains the same. Improvements in the apparatus can be seen in U.S. Pat. Nos. 5,337,599 and 5,101,658. The current model being sold is the JFTOT 230 Mark III, which is described in further detail in the “Jet Fuel Thermal Oxidation Tester—User's Manual”. The determination of the deposits that occur on the heater tube can be made visually by comparing to known color standards or can be made using a “Video Tube Deposit Rater” sold under the Alcor mark.
The determination of the amount of deposits formed on the heater tube at an elevated temperature is an important part of the test. The current ASTM D3241 test method requires a visual comparison between the heater tube deposits and known color standard. However, this involves a subjective evaluation with the human eye. To take away the subjectivity of a person, an electronic video tube deposit rater was developed.
Also, there has been considerable discussion as to the polish or finish of the heater tube. (See U.S. Pat. No. 7,093,481 and U.S. Patent Application Publication No. US 2002/083,760.) The finish of the heater tube is very important in determining the amount of fuel deposits that will form thereon. Therefore, it is important that the quality of the finish on heater tubes made today be consistent with the finish of heater tubes made since 1973.
Once the thermal oxidation stability test has been performed on a batch of fuel, the recorded information and the heater tube are preserved to show the batch of fuel was properly tested. The information that was recorded when testing a batch of fuel is maintained separately from the heater tube itself. This can cause a problem if one or the other gets misplaced or lost. Inaccurate information and/or conclusions occur if the wrong set of data is associated with the wrong heater tube.